1.Field of the Invention
This invention relates to a cable strain relief system, and in particular to a strain relief system for an optical fiber connector.
2. Description of Related Art
Optical fibers are used in a variety of applications ranging from large scale telephonic communication systems to laser surgery, and generally include a transparent core made, for example, of glass surrounded by cladding which is also made of transparent material but has a different index of refraction than the core. The core and cladding combined have a diameter of as little as 125 microns, and are enclosed by a protective flexible jacket having a diameter on the order of 3 mm. Connectors are used to join fibers to each other and to active or passive optical components such as sources, detectors, repeaters, switches, and attenuators.
As is the case with electrical connectors, the point at which the cable enters the connector is subject to excessive stresses when loads are placed on the cable. It is known to relieve the stresses by a strain relief system in the form of a flexible boot. Although the ability of state-of-the-art optical fiber cables to flex without affecting performance has been greatly enhanced recently, there remains a point of flexure beyond which the optical performance of any optical cable seriously degrades.
An ideal strain relief for an optical fiber cable would permit maximum flexibility to a minimum bending radius, within system tolerances, under loads of less than 1 lb. While strain relief systems for various types of cables currently exist which provide flexibility and possibly a minimum bending radius, none allows the optimum combination of maximum flexibility and a controllable minimum bending radius necessary to ensure optical performance of the optical fiber cable.
Examples of prior optical fiber connectors which include strain relief boots of varying degrees of apparent effectiveness are shown in U.S. Pat. Nos. 2,724,736; 4,834,487; 5,030,135; 5,071,219; 5,094,552; 5,138,678; 5,170,452; and 5,202,942. For the most part, these patents give scant attention to the strain relief boot, and none specifically addresses the issues raised below. In U.S. Pat. No. 4,834,487, for example, the only mention of a strain relief is the statement that a " . . . tubular strain relief boot is also shown." Similarly, U.S. Pat. No, 5,202,942 mentions an " . . . elongated, flexible strain relief body portion extending rearwardly therefrom and encircling the cable as known to those skilled in the art", while U.S. Pat. No. 5,138,678 mentions only a " . . . tubular, flexible portion of the strain relief." U.S. Pat. Nos. 5,170,452 and 5,071,219 similarly mention strain reliefs in passing, and do not show any concern for controlling the minimum bending radius while providing maximum flexibility under small loads.
U.S. Pat. Nos. 5,030,135 and 2,724,736 discuss particular strain relief designs, but approach the problem by offering resistance to bending, rather than control of the bend. For example, U.S. Pat. No. 5,030,135 shows a solid tube base with longitudinal, tapering ribs extending outwardly from the base, while U.S. Pat. No. 2,724,736 discloses the use of a coil spring for the same purpose.
Of the above listed patents, only U.S. Pat. No. 5,094,552 addresses the problem of controlling the bending radius in a flexible boot design, but the disclosed design appears to be more complicated than desirable for many applications and does not offer any guidelines for achieving a particular minimum bending radius.